This application is generally related to monolithic coupled-dual resonator crystals and in particular is directed to an improved four frequency measuring process which is useful in accurately measuring three important crystal parameters such as the two resonator frequencies and the synchronous peak separation frequency at any step in the crystal fabrication process after the resonators are formed and up to and including the final test operation.
A typical coupled-dual resonator crystal includes a plate of piezoelectric material having an input electrode and output electrode on one face of the plate and a common electrode on the opposite face of the plate with a first resonator formed by the input electrode and common electrode, and a second resonator formed by the output and common electrodes. Such dual resonator crystal filters are used extensively in electronic applications including IF filters for mobile and cellular radios, for example. Although such devices are mass produced using automated techniques, control of parameters such as electrode locations, thicknesses and spacings are critical, particularly where high center frequencies are required. Such manufacturing techniques require real time accurate measurements of key coupled-dual crystal filter characteristics during the fabrication process as well as at the final test operation.
As indicated in commonly assigned U.S. Pat. No. 4,O93,914 to Peppiatt and Roberts, accurate determination of key characteristics such as the resonant frequency of each of the dual resonators is desirable. Moreover, determination of "synchronous peak separation frequency" (SPSF) is indicated to be of particular significance since it provides a common reference value at a particular point in a process of manufacturing a specific crystal design such that each crystal in the group may be properly evaluated by being mathematically related to standard conditions. These key crystal parameters including synchronous peak separation frequency are calculated based upon four critical frequencies determined by either of two disclosed methods. Both of the disclosed methods involve obtaining four critical frequencies by monitoring one of the two crystal ports while either shorting or effectively open circuiting the second port.
Although the processes disclosed in this commonly assigned patent offer the advantage o requiring the monitoring of only a single port, we have discovered an additional process for obtaining a new set of four critical frequencies which can be used in the calculation of the noted key crystal parameters where both ports must be monitored while producing similar accurate and consistent results.
In brief summary, we have discovered that two of the above noted critical frequencies may be obtained by monitoring a first port of a high frequency monolithic crystal filter with the second port open circuited or with a capacitor connected in parallel therein; whereas, the second two critical frequencies may be obtained by monitoring the opposite or second port with the first port short circuited. Although the latter two frequencies obtained are basically the same as those taught in the aforementioned commonly assigned patent, the first two frequencies measured may be somewhat different than those previously obtained and usually are different unless the coupled-dual crystal is symmetrical, which is not generally the case. Nevertheless, the frequencies obtained would produce correct calculated results for the above noted key crystal parameters in accordance with contemporary measuring standards. Such contemporary measuring standards are discussed in "The four-frequency process for coupled duals using error-corrected S-parameter measurements", by G. E. Roberts, IEEE Trans. on Ultrasonics, Ferroelectrics, & Frequency Control, Vol. 35, No. 3, May 1988, pp. 306-314. Furthermore, since our process can be implemented in a convenient and practical manner using the illustrated structure or known vector network analyzers while producing the noted advantages, substantial improvements in crystal measurements throughout the crystal manufacturing process are possible.